The high cost of drilling wells for the production of oil and natural gas has continued to increase over recent years. This increase in drilling cost is due at least in part to the increasing depth and difficulty of location of remaining hydrocarbon reserves, considering that large and shallow reservoirs around the world have already been exploited. Considering also that drilling costs increase at least linearly with well depth, new hydrocarbon wells are increasingly expensive, especially for those wells in hostile surface or sub-surface environments. The additional factor of market price volatility, especially in recent years, has still further increased the previously significant pressure on producers to drill only where the likelihood of paying production is high.
Confidence in the success of a particular well can be obtained from accurate measurement of the surrounding formation prior to and during the drilling operation. Of course, the seismic survey of the drilling region is an important factor in the success of a well. In addition, accurate assessments of the quality and location of the reservoir and its surrounding sealing formations are also critical in play evaluation, especially in so-called "wildcat" areas that have had little previous drilling activity. These assessments of reservoir and seal quality are generally based upon the analysis of physical samples of the sub-surface formations, such samples known as rock samples.
According to one conventional rock sampling technique, cuttings obtained during drilling are analyzed to determine the composition, porosity, and other lithology parameters of the formation. The analysis of drill cuttings to obtain rock samples is known to have certain drawbacks, however. One such drawback is that the size of the individual cuttings are often so small that one may not be able to accurately determine the petrophysical properties (e.g., porosity, permeability, saturation) of the formation from which they came. Secondly, variations in the circulation time of the cuttings in the drilling fluid prior to sampling cause uncertainty in the determination of the true depth from which the analyzed fragments originated. This uncertainty is compounded by the possible caving in of the wellbore or by mixture of the cuttings from different intervals, such that the sampled cuttings are a mixture of constituents from different depths, rather than discrete lithologies corresponding to formations at different depths. In addition, the drilling fluid causes dispersion of clay-cemented particles so that these particles are disaggregated and lost from the sample; the distribution of rock types remaining in the sample are thus heavily biased toward those types having non-reactive cements. The cumulative effect of these factors makes the analysis of drilling cuttings inexact, at best.
An alternative conventional technique for obtaining rock samples of a wellbore is the use of a wireline sidewall sampling tool after the wellbore has been drilled and logged. Examples of such tools include the CORGUN tool available from Western Atlas International and the CST sidewall core sampler available from Schlumberger Well Services. Additional description of the mechanics of sampling utilizing conventional tools of this type may be found in Wolk, "The Mechanics of Sidewall Sampling", Technical Memorandum, Volume 7, No. 1 (Dresser Atlas, March, 1976), pp 1-12. These conventional sampling tools operate by activating a charge to fire small sampling bits into the surrounding formation; upon retrieval of the tool, the sampled formation remaining in the sampling bits can then be analyzed.
The use of a wireline sampling tool provides useful and relatively accurate information regarding the formations through which the wellbore has been drilled. However, these samples are generally obtained too late to allow for adjustments in the drilling operation itself, and as such have limited impact on the success of the well being drilled.
The significant expense associated with wireline logging for many well operations, particularly in hostile or remote locations, has resulted in the popularity of measurement-while-drilling (MWD) logging, where a tool is included within the drill string to sense downhole conditions and communicate the same to the surface during the drilling operation. Total drilling cost may be greatly reduced by use of MWD logging rather than wireline logs, because the cost of wireline service equipment and personnel at the drilling site is avoided. However, the absence of wireline equipment and service personnel at the MWD drilling site causes the incremental cost of wireline core samples for an MWD well to be prohibitive.
Another drawback to wireline sampling results from degradation of the wellbore over time. One type of time-dependent degradation is enlargement of the hole due to reaction of the formation with the drilling fluid over time, resulting in the swelling, weakening and eventual loss of formation. Other time-dependent degradation mechanisms include dispersion of the formation by the drilling fluid and stress release effects at the wellbore location, each of which also cause loss of formation structure. In addition, invasion of the formation by water-based filtrate will, over time, reduce the oil saturation in the formation, making a later-obtained sample quite inaccurate. "Filter cake" may also build up on the wellbore sidewalls as drilling fluid liquid filters out of the wellbore into the surrounding formation, in which case subsequent sampling of the wellbore sidewalls may retrieve a large amount of filter cake but only a small amount of formation.
As is well known in the art, conventional wireline sampling of the wellbore sidewalls is typically performed at least one or more days after the formation of the wellbore. Because of the time-dependent degradation effects noted above, however, wireline samples may have low sample volumes (e.g., where the formation has been lost or where significant filter cake is formed), or may not accurately portray the true formation composition (e.g., where the saturation is reduced by water-based filtrate).
In addition, conventional wireline sampling cannot be used in wells that are oriented in substantially a horizontal direction, as the shallow angle prevents the tool from being lowered to the depth of interest. Wireline logging and sampling in such wells require the use of coiled tubing to force the tools into the horizontal wellbore; as is well known in the art, the cost of coiled tubing wireline operations is much greater even than conventional wireline logging. The cost of coiled tubing equipment and personnel solely to perform sidewall sampling is therefore especially prohibitive for substantially horizontal MWD (i.e. non-wireline logged) wells.
Another technique for obtaining rock samples is the wellknown coring technique, in which a core drill bit is used periodically during the drilling operation to obtain a core sample of the earth. While providing highly accurate samples, these coring operations are quite expensive considering that trip times of on the order of six to twenty-four hours are required. As a result, conventional coring is limited to portions of certain key reservoir intervals, and important seal and marginal reservoir lithologies thus may not be sampled in early formation evaluation. Core sampling also has limited usefulness in early well decisions.
By way of further background, many types of MWD tools and techniques are well known in the art. Surveys of MWD techniques may be found in Honeybourne, "Measurement While Drilling", Symposium on the 75th Anniversary of the Oil Technology Course at the Royal School of Mines (1988), and in Bonner, et al., "Logging While Drilling: A Three-Year Perspective", Oilfield Review (Elsevier, July 1992), pp. 4-21. Conventional MWD tools are included as a special joint of pipe in the drill string, generally near the drill bit, and include transducers and other sensors for determining downhole conditions during the drilling operation. Examples of the types of information obtainable by conventional MWD include drilling mechanics, drilling direction and inclination, short-normal resistivity and gamma ray detection. In addition, modern MWD tools also utilize logging-while-drilling (LWD) measurements to obtain information similar to that obtained from wireline logs, such as correlation, resistivities, shale volume, porosity, lithology, rugosity, and free gas detection.
One known technique for communicating MWD parameters or alarm conditions to the surface is referred to as stress wave telemetry, where vibrations of the drill string itself communicate the data, and direct communication of electrical signals along wire conductors within the drill string. Another well known technique for downhole-to-surface telemetry is so-called mud-pulse telemetry, where periodic restriction or venting of a flowing drilling mud stream at a downhole location creates pressure variations in the mud stream that are detectable at the surface. An alternative technique for effecting mud-pulse telemetry utilizes the variable rotation of a slotted rotor relative to a slotted stator (such an apparatus referred to as a "mud siren") to produce a frequency modulated signal in the mud stream pressure that is detectable at the surface.
By way of further background, the use of downhole mechanisms to fire projectiles into the formation surrounding the drill bit have been described in U.S. Pat. Nos. 4,004,642 and 4,474,250. These references describe the firing of projectiles into the earth to assist in the excavation of the borehole, and also to produce acoustic signals for detection at the surface from which the lithology of the formation surrounding the bit may be measured. The '642 patent noted above describes the use of ammunition rounds, fired through a gun barrel, as the projectiles.
It is an object of the present invention to provide a method and apparatus for obtaining rock samples from the sidewalls of the wellbore during the drilling operation.
It is a further object of the present invention to utilize a downhole tool to effect the sampling.
It is a further object of the present invention to provide such an apparatus that may be controlled to obtain the necessary samples at selected times, so that accurate reservoir and seal analysis may be obtained in sufficient time to allow adjustment of the drilling operation.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification, in combination with the drawings.